Electrical cables which are used in oil wells must be able to survive and perform satisfactorily under extremely adverse conditions of heat and mechanical stress. Ambient temperatures in wells are often high and the I.sup.2 R losses in the cable itself add to the ambient heat. The service life of a cable is known to be inversely related to the temperature at which it operates. Thus, it is important to be able to remove heat from the cable while it is in its operating environment.
Cables are subjected to mechanical stresses in several ways. It is common practice to attach cables to oil pump pipes to be lowered into a well using bands which can, and do, crush the cables, seriously degrading the effectiveness of the cable insulation and strength. The cables are also subjected to axial tension and lateral impact during use.
It is therefore conventional to provide such cables with external metal armor and to enclose the individual conductors within layers of materials chosen to enhance the strength characteristics of the cable and ensure against insulation destruction, but such measures are sometimes not adequate to provide the necessary protection.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. Typically, the insulation surrounding the conductors in a cable contains micropores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the wall, there is not sufficient time for the intrapore pressure to bleed off. As a result, the insulation tends to expand like a balloon and can rupture, rendering the cable useless thereafter.
In U.S. Pat. No. 4,409,431 in which the assignee is the same as the assignee in the instant invention, there is described a cable structure which is particularly suitable for use in such extremely adverse environments. The structure protects the cable against compressive forces and provides for the dissipation of heat from the cable which is an important feature in high temperature operating environments, for reasons discussed therein, and resistance to decompression expansion of the insulation.
As described in said copending application Ser. No. 291,125, the cable protective structure includes one or more elongated force-resisting members which conform to, and extend parallel and adjacent an insulated conductor comprising the cable. These members are rigid in cross-section to resist compressive forces which would otherwise be borne by the cable conductors. For applications requiring the cable to undergo long-radius bends in service, the elongated support may be formed with a row of spaced-apart slots which extend perpendicularly from the one edge of the member into its body to reduce the cross-sectional rigidity of the member in the slotted areas so as to provide flexibility in the support to large-radius bending about its longitudinal axis.
As described in my copending patent application Ser. No. 390,308 filed June 21, 1982 and assigned to the same assignee as the present invention, for certain service applications, it may be preferred that the electrical insulating sheath on the cable conductor not be in direct contact with the slot openings. This is because the slot openings in the support member may allow highly corrosive materials to gain access to the jacket composition by flowing inwardly through the slots. In addition, the corners formed by the slots may cut into or abrade the underlying cable jacket upon repeated bending of the cable.
The cable protective structure of said copending application Ser. No. 390,308 is made of a composite structure which utilizes an elongated force-resisting member of good thermal conductivity positioned adjacent the insulating conductor sheath. This member comprises a channel member having two substantially parallel elements or legs cantilevered from a transverse or vertical leg and which are slotted laterally to impart the requisite long-radius bending in the plane of the transverse leg. The parallel legs may extend in the same direction from the transverse leg toward an adjacent conductor in which case the channel has a U-cross sectional shape. A smooth, bendable liner may be mounted between the three legs of the channel and the insulation sheath of the adjacent conductor to bridge the slots in the member and thereby protect adjacent insulation from abrasion by the slot edges during bending of the member.
Certain service applications may require even greater resistance to repetitive impacts and high compressive forces than can be withstood by the force-resisting channel member disclosed in my copending application Ser. No. 390,308. Such extreme forces may cause inward bending of one or both of the two parallel channel legs because these forces are typically applied in planes substantially perpendicular to the plane of these legs and eccentric to the planes of attachment between these legs and the central supporting leg of the channel.
For certain applications of flat cable, it may be desirable to increase the thickness of insulation on one or more cable conductors with minimum increase in the cross-sectional dimensions of the cable. For such applications, it would be desirable if the cross-sectional dimensions of the channels could be minimized without adversely affecting the compression and impact resistance of the cable. Other applications for flat cable may require that, in addition to electrical conductors, the cable incorporate one or more hollow conduits for conveying fluids or instrumentation down the cable to, for example, a certain location in a bore hole and provide compression and impact protection to such conduits.